1. Field of the Invention
This invention relates to pulsed-laser lidars and range finders, and more particularly to pulsed-laser ceilometers, which are devices for measuring the height of clouds by measuring the time delay between the transmission of an upwardly projected short laser pulse and the reception of the backscattered return pulse from the underside of the cloud deck.
2. Prior Art Problem
A major problem in the application of laser ceilometers is their deficient performance in measuring cloud height under conditions of poor visibility such as if frequently encountered in fog, rain, heavy haze, sleet or snow. Not only does the poor visibility lead to a reduced signal level from the clouds but, more importantly, the backscattered signal from the visibility-reducing suspended particles can be large compared to the backscattered signal from the cloud deck, and can thus confound detection of the latter. A similar problem attends the use of other forms of pulsed-laser lidar, sensing targets through turbid media, but for simplicity the following disclosure will refer to pulsed-laser ceilometers.